1. Field of the Invention
The present invention relates to a gas oil, such as a diesel fuel, composition having improved lubricity and a low sulfur content which can reduce wear of metallic surfaces the composition contacts; a gas oil additive to be used for the gas oil composition; and a process for preparing the gas oil additive.
2. Description of the Related Art
In recent years, environmental pollution by nitrogen oxides (NO.sub.X), sulfur oxides (SO.sub.X) and particulate matters each contained in an exhaust gas coming from diesel cars has arisen as a problem. To describe specially, it has been a problem that acid matter formed by the combustion of sulfur components contained in gas oil as a fuel for engines causes corrosion and wear of metal, and acid rain. Up to now, nitrogen oxides and sulfur oxides contained in exhaust gas have been regulated, and a new regulation with respect to particulate matter has been added.
It has been firstly proposed as a countermeasure for this regulation with respect to the particulate matter to reduce the sulfur oxides contained in exhaust gas, in other word, to cut down a sulfur content of gas oil in stages. Further, the amendment (1990) of the Clean Air Act in U.S.A. provides that the sulfur content of gas oil as a quality standard must be 0.05% by weight or less. In Japan, it is anticipated that there will be taken a phased countermeasure to restrict the sulfur content among the quality standard of gas oil in Japan firstly to 0.2% by weight or less, and further to 0.05% by weight or less within this century. Under such circumstances, patent applications each disclosing a process for preparing a gas oil for diesel engines having a low sulfur content have been filed, and, further, gas oils each having a low sulfur content have already been put on the market in, e.g., Europe and U.S.A.
However, reductions in the sulfur content of the gas oil has caused a new problem of fuel pump seizures due to a reduction of the lubricity of the gas oil (see SAE Paper 942016). By using a gas oil having a low sulfur content and a low aromatic-compounds content as an environment-protecting fuel, in fact, several thousands of cars have encountered troubles caused by a lack in the lubricity of the gas oil in Sweden in Europe. While, in Japan, gas oils for diesel engines each having a low sulfur content are not presently popular. However, also in Japan, it is anticipated that the employment of gas oils for diesel engines having a low sulfur content in the near future will cause the problems of seize of a fuel pump and wear of metallic surfaces. International Publication No. WO 94/17160 (published on Aug. 4, 1994) and European Patent Publication-A No. 0608149 (published on Jul. 27, 1994) disclose fuel compositions each comprising an ester of a carboxylic acid having 2 to 50 carbon atoms with an alcohol, which are useful to improve lubricity and reduce wear in diesel engines. In thses documents, glycerol monooleate and diisodecyl adipate are cited as examples of such esters. However, the fuel compositions are expensive, and the objects of the inventions described in the above documents, i.e., improvement of lubricity and reduction of wear in diesel engines, are not sufficiently attained by the fuel compositions.